1. Field of the Invention
This invention relates to an optical fiber which includes a plurality of holes around a core thereof, an end face structure that an end face of the optical fiber is connected to another optical fiber, and an optical connector to which an end portion of the optical fiber is bonded and fixed and which has a high reliability by sealing the holes at an end face of the optical fiber.
2. Description of the Related Art
FIG. 4 is a sectional view showing a holey optical fiber (Hole Assisted Fiber: HAF) having 6 holes.
As shown in FIG. 4, the HAF 40 includes a core 41, a clad 42 covering the core 41, and a plurality of holes 43 (e.g., 6 holes in FIG. 4) formed in the clad 42 around the core 41.
For example, the HAF 40 has a core diameter of 10 μm and a clad diameter of 125 μm. Further, Germanium is added to the core 41 similar to a typical (solid) single mode fiber (SMF), and the core 41 has a relative refractive index difference of 0.35% with respect to the clad 42 made of a pure silica glass. Around the core 41, the holes 43 having an internal diameter of about 10 μm are formed along with an entire length of the optical fiber at equal intervals with respect to a circumferential direction.
A feature of the HAF 40 is to have a high optical confinement into the core 41, since a refractive index of the holes 43 is almost 1.0 and an effective relative refractive index difference is extremely greater than that of a tipical SMF. Further, if the relative refractive index difference is great, a loss which arises if the HAF 40 is bent, is extremely small.
An optical fiber having holes such as a HAF 40 is “connectorized” (e.g., an optical fiber end face is fixed to an optical connector) after the holes of the optical fiber end are sealed. As a method therefor, there has been a method sealing by heating, and a method sealing by injecting an ultraviolet (UV) curing resin (e.g., disclosed in JP-A-2002-323625, hereinafter “patent document 1”).
In more detail, after holes are destroyed (e.g., crushed) and sealed by locally heating an optical fiber by using a heating means (e.g., heater, or discharging electrode) while reducing a pressure from an edge of the holes by a vacuum pump, the holes are destroyed in the same manner at a location (the other end of the optical fiber) which is distant therefrom by a required length, and then both ends of the optical fiber are connected to desired connector components.
On the other hand, concerning a sealing method by UV curing resin, after a liquid UV curing resin (resin precursor) is injected from an optical fiber end face, the liquid resin precursor in the holes is hardened by irradiation of ultraviolet rays. In this case, a refractive index of the UV curing resin is greater than that of a material constituting a clad. After hardening, an optical fiber end portion filled by the resin is inserted into a hole of a ferrule, and fixed to the ferrule by an adhesive. Then, the optical fiber protruding from a ferrule end face is grinded (polished), and an optical connector process is completed.
A viscosity of the UV curing resin filled in the holes is more than 10 Pa·s, and preferably 60 Pa·s (e.g., as disclosed in JP-A-2006-126720, hereinafter “patent document 2”). With regard to an optical fiber of the patent document 2, a grinding process after hardening is not considered. Further, if a length (resin filling length) filled by a silicon adhesive as a UV curing resin is great, an internal bubble may occur during hardening. Consequently, the viscosity of the resin precursor is set relatively high so that the silicon adhesive can be controlled well within a relatively short range (resin filling length).
However, as shown in FIG. 5, the optical fiber disclosed in patent document 1 is a photonic crystal fiber (PCF) which has holes 52 which-have a diameter of several μm and are formed around the center of the clad 51 having a uniform refractive index without the center thereof, so as to have a honeycomb structure and have layers equal to or more than 3 (e.g., 4 layers in FIG. 5), and in which the center surrounded by the multiple holes 52 is core 53. Since the PCF has a multi-layer space of the clad (multi-layer structure of a refractive index), a photonic band gap (PBG) as a forbidden band of a light is formed, and a light is confined in the core by the PBG.
The above-mentioned optical fiber (e.g., patent document 1) has the refractive index of the UV curing resin filled in the holes greater than that of a material constituting the clad part. Although the above-mentioned sealing method by using the UV curing resin is effective for PCF 50 having holes which are formed so as to have a honeycomb structure and have layers equal to or more than 3, if in the HAF 40 shown in FIG. 4 the refractive index of the UV curing resin filled in the holes is greater than that of the clad, a part around the core 41 becomes a side core, light leaks from the center core 41, and a mode field diameter, which is an effective propagation area of a light, becomes extremely large. Thus, a part filled by the UV curing resin is considered as a multi-mode fiber, a transmission loss becomes large, and when butt-connecting to an SMF, a connection loss becomes large since the mode field diameter does not match that of the SMF.
Additionally, although a resin filled in a plurality of holes is required to have several properties in order to process an end portion of an optical fiber having the holes around a core so as to be an optical connector, a conventional optical fiber (e.g., patent document 1) has not been considered with respect to properties of a UV curing optically-transparent resin filled in the holes (e.g., parameters other than a refractive index).
Thus, when an end portion of an HAF, in which a UV curing resin is filled, is processed (e.g., polished, connected to another optical fiber, optically-connectorized, etc.), the end portion may be damaged or the filled UV curing resin may be dented (e.g., sunken) at the end face.